Hi @Max Johnson ,
It seems that you have some sites that are still using the SharePoint 2010 experience, which is not compatible with the SharePoint 2016 farm.
You must upgrade all SharePoint 2010 mode sites to 2013 mode (that is, compatibility level 15) on the existing 2013 farm before you mount the database on the new SharePoint 2016 farm. You can use the following PowerShell command to check the compatibility level of a site collection:
Run below powershell to check if all site collection are in the same version:
get-spsite
For example, here are my test results:
If there are sites with a collection compatibility level other than 15, run below script to upgrade Sites:
$webApps = Get-SPWebApplication -Identity "http://servername:12345/"
$sites = $webApps.Sites>
foreach($site in $sites)
{ Upgrade-SPSite -Identity $site -VersionUpgrade -Unthrottled $site.Dispose() }
Hope the information can help you.
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